ok.. but it seems confusing Your answer is correct and I know that but I don't know how u got it.

I was scared of this...ok:

Work through my solution step by step by looking at the diagram...

Step 1: I found the area of triangle XBY (the triangle surrounded by the points X, B and Y). This area is the area of the whole thing (this is not XYC but the areas are awkardly the same...).

Step 2: I found the area of the trapezium AXCD (you see I'm finding the area of everything EXCEPT the area of XYC). This turned out to be the area of the whole thing. This can be said as the area of the whole thing, to make it easier later.

Step 3: I added the two areas together which is . That is the area of EVERYTHING but the pane of glass we are cutting out.

Step 4: We can now work out the area of pane of glass by . That means that fraction area of the pain of glass is that of the rectangular area of the pane of glass.